I don't know what caused the black tips, but it wasn't the rain the previous night. Fungus diseases take a few days to a week to show symptoms, but black tips probably aren't caused by fungus. Over fertilizing is one possible cause.

Cornell mixture shouldn't go bad in storage.

Sounds like you should find out what varieties you neighbor has. Or keep admiring them vocally, and maybe she'll set a rock on a weak shoot and root you one.

Now I think I am either getting insane or have hallucinations or what. When I looked at it the next day, they looked normal again. I have no explanation for that. Only that the leafs might have been so soaked with water, and that it actually accumulated in the tips and discolored it temporarily. Like everything looks darker when it is wet. But I swear it looked BLACK. The roses are sitting next to the house and we don't have a gutter, so they get the full shower from the roof.
I will make a picture if it happens again. I am not ready for the nuthouse yet. This is so weird.
BTW, I took a closer look at my neighbor's roses and they have some blackspot, but appearantly not enough to have an impact on them. From what I have read here so far, some roses are just like that.